By default when I compose an e-mail in GMail, the spell checker is active.
To disable it, I need to activate context menu (Right Click) and select "Check Spelling" option to toggle off. Is there any way to disable it on a permanent basis, so always when a I start composing an e-mail no spell checking is performed ?

As @w3d pointed out, is not an issue with Gmail configuration, but my browser's. I'm using Firefox 14.0.1 and the method to disable the spell checker (in order to avoid spell checking in Gmail composer) is deselecting the option in Tools -> Options -> Advanced -> "Check my spelling as I type".